The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing the migration of charged pigment particles in a solvent, preferably in a dielectric solvent. More specifically, an electrophoretic fluid comprising charged pigment particles dispersed in a dielectric solvent is enclosed between two electrode plates. At least one of the electrode plates is transparent and such a transparent plate is usually the viewing side. When a voltage difference is imposed between the two electrode plates, the charged pigment particles migrate by attraction to the electrode plate of polarity opposite that of the charged pigment particles. Thus, the color showing at the viewing side may be either the color of the dielectric solvent or the color of the charged pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite electrode plate, thereby reversing the color. Alternatively, two types of pigment particles of different colors and polarities may be dispersed in a solvent. In this case, when a voltage difference is imposed between the two electrode plates, the color showing at the viewing side would be one of the two colors of the pigment particles. Reversal of plate polarity will cause the two types of pigment particles to switch positions, thus reversing the color.
Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing time.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD [M.A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)], the microencapsulated EPD (U.S. Pat. Nos. 5,961,804, 5,930,026, and 7,184,197. and the total internal reflection (TIR) type of EPD using microprisms or microgrooves as disclosed in M.A. Mossman, et al, SID 01 Digest pp. 1054 (2001); SID IDRC proceedings, pp. 311 (2001); and SID'02 Digest, pp. 522 (2002).
An improved EPD technology was disclosed in U.S. Pat. Nos. 6,930,818, 6,859,302 and 6,788,449, the contents of all of which are incorporated herein by reference in their entirety. The improved electrophoretic display comprises isolated display cells formed from microcups which are filled with charged pigment particles dispersed in a dielectric solvent. To confine and isolate the electrophoretic fluid in the microcups, the filled microcups are top-sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from the group consisting of thermoplastics, thermoplastic elastomers, thermosets and precursors thereof.
The U.S. patents identified above also disclose a roll-to-roll process for manufacturing electrophoretic displays. With a roll-to-roll manufacturing process, in-line testing and inspection of the elelctrophoretic display panel produced is highly desirable.
Currently, inspection of an electrophoretic display panel is often carried out by applying a temporary conductive layer to the display panel. The temporary conductive layer is on the opposite side of one of the two electrode plates already in place. When a voltage difference is applied between the temporary conductive layer and the electrode plate, the performance of the display panel (i.e., switching of the charged pigment particles) can be visually inspected. The temporary conductive layer, however, has to be removed before the second electrode plate is applied, to complete the assembly. The use of a temporary conductive layer therefore is not an efficient and cost-effective way for testing and inspection.